The present invention relates to tandem mass spectrometers for selected ion fragmentation studies and to ion fragmentation processes for use in such spectrometers.
Selected ion fragmentation mass spectrometers have recently been developed wherein a tandem mass spectrometer is used to create ion species from a sample, select one individual ion species, fragment it such as by collision induced dissociation (CID), and obtain the mass spectrum of the fragments. This analysis has been found to be useful for such purposes as structure elucidation, mixture analysis, and determination of isotopic labeling. The following chart summarizes this analysis process:
__________________________________________________________________________ : Mass : Frag- : Mass : Sample : Ionization : Sep'n : mentation : Sep'n : Detection __________________________________________________________________________ : : : : : : : : : : ##STR1## : ##STR2## : ##STR3## ##STR4## ##STR5## STRUCTURE DATA ##STR6## ##STR7## ##STR8## ##STR9## ##STR10## MIXTURE DATA : : : : : __________________________________________________________________________
Considering the above chart, in structure elucidation, the pure sample (M) is ionized by a hard ionization process such as electron impact, producing parent and fragment ions (M.sup.+ and fragments Q.sup.+, R.sup.+). Any one of these ions (say, Q.sup.+) may be selected by the first mass separation, and further fragmented to produce fragment ions (A.sup.+, B.sup.+, C.sup.+). The resulting mass spectrum can then be obtained by scanning the second mass separation to pass one ion at a time (A.sup.+, B.sup.+ or C.sup.+).
In mixture analysis, the multicomponent sample (components K, M, N) is subjected to soft ionization, such as provided by chemical ionization, to produce primarily the parent ion of each component (K.sup.+, M.sup.+, N.sup.+). The first mass separation will allow one parent ion (say, M.sup.+) at a time to pass into the fragmentation region where fragments or daughters are formed (Q.sup.+, R.sup.+). The mass spectrum can then be obtained by scanning the second mass separation to pass one ion at a time (M.sup.+, Q.sup.+ or R.sup.+). The first stage of mass separation, then, is used to separate the components of the mixture; the second stage to obtain mass spectra of the individual components.
Heretofore, instruments used to perform these analyses have generally been of the type wherein a magnetic mass selector and an electrostatic energy selector are coupled in tandem. The energy selector produces an ion kinetic energy separation which is interpreted to provide the fragmentation mass spectrum. Such an instrument may be realized by exchanging the source and collector on most commercially available double focusing mass spectrometers. Additionally, reversed sector double focusing mass spectrometers are commercially available. Various names which have been applied to this general technique are mass-analyzed ion kinetic energy spectrometry (MIKES), direct analysis of daughter ions (DADI), and collisional activation mass spectrometry (CAMS). All of these names refer to the technique in which kinetic energy analysis in the second (electric) sector of a reversed sector double focusing mass spectrometer is used to provide data on metastable or CID ions. For convenience, all of these instruments are referred to hereinafter as "prior tandem instruments".
One goal is to develop an automated mass-spectrometry system, including a data processing unit for controlling the operating parameters of the various devices in the system and interpreting the results, in which an unknown sample may be inserted, with analysis results being printed out by the machine with a minimum of operator intervention. Such an automatic approach is illustrated generally by U.S. Pat. Nos. 4,008,388--McLafferty et al and 4,084,090--Boettger et al.
Again considering terminology, other names which may be applied to a "mass selector" as referred to above are "mass filter" and "mass analyzer". In any such device, particles are in effect separated into a spectrum according to mass or charge-to-mass ratio, and the particles comprising a particular spectral component are selected for further analysis. The selection is accomplished by an exit aperture. Rather than physically move the exit aperture to examine or scan the various spectral components, the position of the spectral distribution is varied by controlling such parameters as electric or magnetic field strength or the ion kinetic energy within the mass selector. Thus the mass selector may be viewed as a filter which passes only a particular spectral component depending on the controlled parameter. A mass scan or analysis is accomplished by varying the controlled parameter as a function of time to pass different mass spectral components, while collecting or detecting the output of the filter.
While results have been impressive with prior tandem instruments, there are a number of disadvantages. A first general disadvantage is a lack of convenient real time control. Especially in a tandem system, it is desirable to scan the mass spectra both before and after fragmentation in real time as rapidly as possible and to be able to step quickly from one mass selection to another. One primary impediment with prior tandem instruments to rapid scanning of a mass spectrum is the relatively long time required to change the magnetic field strength of the mass analyzer due to the inductive nature of the electromagnets employed. Alternatively, if scanning is accomplished by changing ion kinetic energy, processes in the CID region are affected and compensating voltage adjustments are required in the ion kinetic energy selector. Additionally, as pointed out next, it is difficult to rapidly vary the degree of fragmentation, a potentially valuable capability.
A second general disadvantage, as suggested above, relates to the fragmentation process. Two basic fragmentation processes have been used in prior tandem instruments: (1) spontaneous fragmentation of metastable ions, and (2) collision induced dissociation (CID) at relatively high ion kinetic energies above 1 keV.
The heretofore known CID process in such systems is relatively inefficient. The fragmentation efficiency is typically 10% or less. Ion losses due to scattering or other causes may be 90%, with a resultant collection efficiency of 10%. Taking the product of the fragmentation efficiency and the collection efficiency, the overall CID efficiency is then one percent or less. Efficiency may even go as low as one particle in 10.sup.5. One reason for this low fragmentation/collection efficiency is that with the relatively high collision energies employed, the factors favoring collision and fragmentation also increase scattering and other losses in the ion beam.
Further, as already mentioned, with both of these prior art fragmentation processes it is difficult to vary the degree of fragmentation. The degree of metastable fragmentation in prior tandem instruments can be varied only by changing that portion of the ion flight time which falls in a field free region. This can be done either by changing the length of the field free region (which requires mechanical modification) or by varying the accelerating voltage, and hence ion velocity. Varying the accelerating voltage is not easily done, however, since it requires proportional changes in the magnetic field and the electrostatic analyzer voltage.
It is also difficult to vary the degree of CID fragmentation in prior tandem instruments since the degree of fragmentation at accelerating voltages greater than approximately 1 kV is quite insensitive to changes in the collision gas pressure or the accelerating voltage.
Moreover, these first and second general disadvantages, namely the lack of fast real time control over the mass selection and the fragmentation process inefficiency together make sample size requirements higher than they might otherwise be. Similarly, they make the process more difficult to effectively automate.
A third general disadvantage with prior tandem instruments is in the area of resolution. As previously mentioned, these instruments use an electrostatic analyzer for the second stage of mass analysis. While the kinetic energy resolution of the electrostatic analyzer itself is not a limiting factor, as a consequence of using a kinetic energy analysis and interpreting the energy analysis to determine mass, the mass resolution is limited to approximately one part in 80 to one part in 300. During either collision induced dissociation (CID) or metastable dissociation, there is a kinetic energy release. This causes an energy spread in the resultant ion peaks. In addition, CID imposes an extra broadening due to energy spread caused by scattering.
This limited resolution is particularly a problem in isotopic analysis, in which unit mass resolution throughout the entire mass range is required to allow adjacent isotopic peaks to be determined. The kinetic energy broadening can be made relatively less by increasing the accelerating voltage above the usual 3-7 kV. However, accelerating voltages even in the range of 10-20 kV are quite difficult to handle instrumentally. So long as ion energy analysis is used to infer ion mass, this problem will not be solved for the prior tandem type instruments.
The first and third general disadvantages mentioned above, namely the lack of fast real time control over scanning of the mass spectra and resolution shortcomings, of the prior tandem instruments could be overcome by employing a quadrupole-type mass filter or analyzer. The particular ion mass passed by such a filter can be very rapidly changed because such devices are electrodynamic only, and do not have the large inductance of electromagnets. Therefore a mass spectrum may be very rapidly scanned. An early disclosure of such a mass filter or mass analyzer is in Paul et al U.S. Pat. No. 2,939,952. More recent descriptions may be found in G. Lawson and J. F. J. Todd, Chem. Br., vol. 8, p. 373 (1972); and in the book "Quadrupole Mass Spectrometry and Its Applications," by Peter H. Dawson. While the most widely used arrangement is the basic quadrupole having exactly four electrodes, it will be appreciated that more or fewer electrodes may be employed if desired. Accordingly, the term "quadrupole-type device" as used herein is intended to cover all such devices (monopoles, octopoles, etc.) as operate on the principles as described in the above mentioned Paul et al patent, and other references.
However, despite the advantages which would accrue with the use of a quadrupole-type mass filter, direct substitution of quadrupole-type mass filters or analyzers for the analyzers in prior tandem instruments is not feasible because the electric forces employed in a quadrupole, being relatively weak compared to magnetic bending forces, are effective at ion kinetic energies up to only about 30 eV. Such low ion kinetic energies are incompatible with the relatively high ion kinetic energies needed for fragmentation by previously known CID processes, which as mentioned above, occur at energies about 1 keV.
It should be noted that certain tandem quadrupole mass spectrometers are prior art with respect to the present invention. For example, tandem quadrupole mass spectrometers for the study of ion-molecule reactions are described in the following three literature references: J. F. Futrell and T. O. Tierman in "Ion-Molecule Reactions", J. L. Franklin, Ed., Plenum Press, New York, N.Y., 1972, Chapter 11; C. R. Iden, R. Liardon, and W. S. Koski, J. Chem. Phys., vol. 56, p. 851 (1972); and T. Y. Yu, M. H. Cheng, V. Kempter, and F. W. Lampe, J. Phys. Chem., vol. 76, p. 3321 (1972).
In addition, and with particular relevance to the present invention, as described in the following two literature references a center RF-only quadrupole has been added to tandem quadrupole mass spectrometers for the purpose of photodissociation studies: M. L. Vestal and J. H. Futrell, Chem. Phys. Lett., vol. 28, p. 559 (1954); and D. C. McGilvery and J. D. Morrison, Int. J. Mass Spectrom. Ion Phys., vol. 28, pp. 81-92 (1978). Similarly, a center RF-only quadrupole has been added for metastable ion studies as described in the following literature reference: U. von Zahn and H. Tatarczyk, Phys. Lett., vol. 12, p. 190 (1964).
However, despite the prior art tandem quadrupole mass spectrometer systems referred to above, prior to the present invention all of the reported selected ion fragmentation work has been performed on reversed-sector MIKES instruments. Six literature references reporting such results are: R. W. Kondrat, and R. G. Cooks, Anal. Chem., vol. 50, p. 81A (1978); T. L. Kruger, J. F. Litton, R. W. Kondrat, and R. G. Cooks, Anal. Chem., vol. 48, p. 2113 (1976); H. H. Tuithof, Int. J. Mass Spectrom. Ion Phys., vol. 23, p. 147 (1977); U. P. Schlunegger, Angew. Chem., Int. Ed., Engl., vol. 14, p. 679 (1975); K. Levsen and H. Schwarz, Angew. Chem., Int. Ed. Engl., vol. 15, p. 509 (1976); and F. W. McLafferty, P. F. Bente, III, R. Kornfield, S. C. Tsai, and I. Howe, J. Am. Chem. Soc., vol. 95, p. 2120 (1973).